1. Field of the Invention
This invention relates to aircraft instrumentation and, more particularly, to devices extending into the air stream for sensing various pressures which are used to develop derived air data for instrument readout.
2. Description of the Prior Art
It has long been customary to use probes which project from or are mounted to the external surfaces of an aircraft to make various air data measurements which are useful in providing certain instrument displays to a pilot. These probes have been of various types, ranging from the early Pitot tube to multi-aperture, multi-purpose probes mounted at selected points on the aircraft. One particular probe which is used in certain trans-sonic and supersonic aircraft is disclosed in the De Leo et al U.S. Pat. No. 3,318,146. This is a generally cylindrical probe with a hemispherical forward surface and is designed to be mounted to extend laterally from the side of an aircraft. Other probes of a similar type may have a cylindrical body with a conical or ogival forward surface.
The De Leo et al probe contains a forward facing total pressure (P.sub.T) port, located on the central axis of the probe, and four other ports distributed every 90.degree. about the central axis. A first pair of these ports is located in a central vertical plane, the other pair being located in a central horizontal plane. In this probe, each of the ports is connected to its own separate conduit which leads to a corresponding sensor coupled to a computer in which appropriate scale factors can be supplied for developing desired output information. In straight and level flight, the pressure at the central axis port is total pressure, equal to the normal Pitot or stagnation pressure of the aircraft. The pressure mmeasurement at the central port admittedly varies with aircraft attitude. In this particular instrument, the off axis ports are used to derive measurements of angle of attack (.alpha.) and angle of side slip (.beta.). With the five pressure measurements available from this probe, angle of attack, angle of side slip, total pressure, static pressure and Mach number can be derived. Mach number is the ratio of the speed of the aircraft to the speed of sound in the fluid medium and is a function, for the De Leo et al probe, of the average pressure between the off-axis ports in the horizontal plane divided by the total pressure. The relationship between the pressures and Mach number can be expressed as: EQU For subsonic speeds, P.sub.T /P.sub.S =(1+0.2M.sup.2).sup.3.5 ( 1) EQU For supersonic speeds, P.sub.T /P.sub.S =(1.2M.sup.2).sup.3.5 [6/(7M.sup.2 -1)].sup.2.5 ( 2)
where M is Mach number, P.sub.T is total pressure and P.sub.S is static pressure. P.sub.S can be calculated when P.sub.T and M are known or it can be measured independently by static pressure ports.
While the scale factor and certainty of air flow angle detection of hemisphere probes like that of De Leo et al are superior in performance, the total pressure measurement varies with air flow angle approximately in proportion to a cosine squared function. It is preferable that the total pressure measurement be insensitive to angle of air flow to at least +/-20 degrees.
An alternative probe configuration utilizes the angle-insensitive ogival total pressure source in conjunction with angle-of-attack sensing ports which are back on a second concical segement of the cylindrical afterbody. Probes of this type are disclosed in the De Leo et al U.S. Pat. Nos. 4,096,744 and 4,378,696. The characteristic of this differential pressure to angle of attack is much weaker at low angles and becomes non-linear at high angles of attack with a consequent loss of predictability.